This invention relates to technology for removing unwanted metal from semiconductor wafers. More particularly, it pertains to chemical dilution modules for preparing etchant chemicals for use in etching away the unwanted metal.
Damascene processing is a method for forming metal lines on integrated circuits. It is often a preferred method because it requires fewer processing steps than other methods and offers a higher yield. In Damascene processing, as well as other integrated circuit manufacturing processes, the conductive routes on the surface of the circuit are generally formed out of a common metal, traditionally aluminum. Copper is a favored metal because of its higher conductivity and electromigration resistance when compared to aluminum, but copper presents special challenges because it readily diffuses into silicon oxide and reduces its electrical resistance at very low doping levels.
During integrated circuit fabrication, conductive metal is needed on the active circuit region of the wafer, i.e., the main interior region on the front side, but is undesirable elsewhere. In a typical copper Damascene process, the formation of the desired conductive routes generally begins with a thin physical vapor deposition (PVD) of the metal, followed by a thicker electrofill layer (which is formed by electroplating). The PVD process is typically sputtering. In order to maximize the size of the wafer's useable area (sometimes referred to herein as the "active surface region") and thereby maximize the number of integrated circuits produced per wafer), the electrofilled metal must be deposited to very near the edge of the semiconductor wafer. Thus, it is necessary to allow physical vapor deposition of the metal over the entire front side of the wafer. As a byproduct of this process step, PVD metal typically coats the front edge area outside the active circuit region, as well as the side edge, and to some degree, the backside. Electrofill of the metal is much easier to control, since the electroplating apparatus can be designed to exclude the electroplating solution from undesired areas such as the edge and backside of the wafer. One example of plating apparatus that constrains electroplating solution to the wafer active surface is the SABRE.TM. clamshell electroplating apparatus available from Novellus Systems, Inc. of San Jose, Calif. and described in pending U.S. patent application Ser. No. 08/969,984, "CLAMSHELL APPARATUS FOR ELECTROCHEMICALLY TREATING SEMICONDUCTOR WAFERS" naming E. Patton et al. as inventors, and filed Nov. 13, 1997, which is herein incorporated by reference in its entirety.
The PVD metal remaining on the wafer edge after electrofill is undesirable for various reasons. One reason is that PVD metal layers are thin and tend to flake off during subsequent handling, thus generating undesirable particles. This can be understood as follows. At the front side edge of the wafer, the wafer surface is beveled. Here the PVD layers are not only thin, but also unevenly deposited. Thus, they do not adhere well. Adhesion of subsequent dielectric layers onto such thin metal is also poor, thus introducing the possibility of even more particle generation. By contrast the PVD metal on the active interior region of the wafer is simply covered with thick, even electrofill metal and planarized by CMP down to the dielectric. This flat surface, which is mostly dielectric, is then covered with a barrier layer substance such as SiN that both adheres well to the dielectric and aids in the adhesion of subsequent layers. Another reason to remove the residual PVD metal layers in the wafer edge area is that the barrier layers underneath them are also thin and uneven, which may allow migration of the metal into the dielectric. This problem is especially important when the metal is copper.
To address these problems, semiconductor equipment may have to allow etching of the unwanted residual metal layers. Various difficulties will be encountered in designing a suitable etching system.
Depending upon the type of metal to be removed and the characteristics of the etching system, some liquid etchant compositions are appropriate and others are not. In general, the liquid etchant should etch the unwanted metal rapidly at room temperature. But, it should not aggressively attack the mechanical and electrical components of the etch system. In addition, it should not liberate dangerous, gaseous by-products during the etching reaction. For example, nitric acid should be avoided because it liberates nitric oxide during reaction with copper. Still further, the components of the liquid etchant should include only those materials readily available in normal integrated circuit manufacturing facilities. Other beneficial properties of a liquid etchant include a long shelf life (preferably without stabilizers) and a consistent etching rate over time.
The liquid etchant should have physical properties compatible with the etching system. The viscosity and density should allow delivery onto the semiconductor wafer in a desired flow regime (e.g., a viscous flow regime). In addition, the liquid etchant must flow in a continuous stream without abrupt interruptions caused by bubbles and the like. Such discontinuities cause inconsistent application of the etchant on the wafer and lead to inconsistent chemical etching. For example, bubbles in the liquid etchant can generate a scallop pattern on the semiconductor wafer. They may also cause the etchant to spray onto the active area of the semiconductor wafer.
In view of these considerations, the choice of an appropriate chemical etchant is a non-trivial problem. It must be tailored to the particular problems and considerations inherent in an etching module and the underlying chemical etching process. Further, the liquid etchant ultimately chosen introduces a host of special problems related to the preparation and handling of the liquid etchant prior to and during use. These problems require creative solutions. The present invention addresses all of these concerns.